The present invention relates to a protection system arranged to prevent a large trans-membrane pressure difference over a pressure sensitive membrane (4) fitted in a pressure resistant chamber, where it forms a physical barrier between a liquid and a gas mixture carried through the chamber.
The present invention has been developed specifically for the protection of membranes used for natural gas sweetening (mainly C02 removal, but also for the removal of H2S and other sour gas components) and for natural gas dehydration (H20 removal). Such membranes are generally formed as cylindrical tubes. The gas is normally directed along the exterior face of the membrane, and sour gas components diffuses through the membrane, being absorbed by the liquid (usually amine) flowing axially through the interior of the membrane. The ability of the liquid to absorb the sour gas components increases greatly with increasing pressure. Continuous removal of sour gas components is effected by reducing the pressure of the liquid after passage through the membranes, in order to release the sour gas components to the surrounding atmosphere. The pressure of the natural gas is typically of the order 30-100 bar. As the membranes are easily damaged even by moderate trans-membrane pressure differences, it must be possible to control the pressure of the liquid very closely. Membranes used for this purpose may be damaged even at pressure differences at low as +/xe2x88x92 1 bar.
Conventionally, the method employed to secure against harmful pressure differences includes providing a buffer tank, in which the liquid and the natural gas is in open contact. The variations in pressure are thus taken up by liquid flowing into or out of the buffer tank. This conventional system will require a buffer tank of relatively large volume and weight, due to being designed for high pressure. The response time by shut-off due to excessive increase of pressure difference is relatively long. The pressure difference over the membrane may then at all times be kept low, by installing a simple control device to ensure reduced output from the pump upon an increase in buffer tank liquid level, and vice versa.
The objective of the present invention is to provide a protection system that is capable of protecting the membranes against harmful pressure effects. Furthermore, it is an objective of the present invention to provide a protection system that seeks to put the plant back into normal operation as soon as conditions permit this.
This and other objectives that will become clear from the description of preferred embodiments of the invention.
These objectives are achieved by a protection system arranged to prevent a large trans-membrane pressure difference over a pressure sensitive membrane fitted in a pressure resistant chamber, where the membrane forms a physical barrier between a liquid and a gas mixture carried through the chamber, which is characterised in that the protection system comprises two shut-off valves provided in the liquid inlet and outlets lines respectively, a buffer tank provided in the line between one of the shut-off valves and the pressure resistant chamber, a pressure equalising valve provided between the gas inlet line and the buffer tank and a pressure sensing valve for measuring the pressure difference over the membrane and which activates the shut-off valves once this pressure difference exceed a predetermined value.
The cross section of the valve seats of the shut-off valves is 2 to 5 times larger than the effective flow area, to give the valve body a short travel distance from the open to the shut position.
The buffer tank is arranged to maintain a stable liquid level and the buffer tank comprise a floating body that co-operates with a valve arrangement which, when the liquid level in the buffer tank is too low, opens a channel for venting of gas from the buffer tank, and also opens to a channel for topping up to the liquid buffer volume, and which, when the liquid level is high, opens a channel for dumping of liquid to a reservoir at a lower pressure.
The shut-off valves have a typical response time from fully open to fully closed position of less than 1 second.
The invention will be described in more detail in the following with reference to the accompanying drawings.